it's very popular these days that younger date older. I think that she actually tries to tell you that obviously there is a problem, but she doesn't want any criticism so when you suspect something, she takes a step back, denies everything. OR, it could be just that she wants someone to listen to her complaints every time they fight or disagree or whatever, and you are her sister. Have you tried speaking to her? calmly, without any criticism, so she understands that you are on her side.
Well, technically she is an adult, and can make her own decisions, but have you told your parents about your suspicions? She may be too terrified to admit that he's abusing her, or may even be blaming herself for it. Unfortunately, most police departments won't respond based on suspicion alone. Usually, there has to be physical evidence, otherwise it's he said/she said, and the complaint usually has to come from the victim. I'm not sure about some departments, but this is a general protocol. You could try calling you're local women's shelter to see what they recommend.